This invention relates to a hybrid drive device combining an engine and an electrical motor as the prime mover for a vehicle.
A hybrid drive device is known in which an electrical motor is driven to operate a vehicle with the electrical power generated by a generator which is driven by an engine. A hybrid vehicle improves exhaust emission performance and vehicle fuel economy by use of this type of drive device.
Many such hybrid drive devices use a secondary battery as a storage device charged by the excess electrical power of the generator. However since a secondary battery displays poor charging/discharging characteristics, desired fuel economy performance resulting from the regeneration of braking energy is not realized. Regeneration of braking energy is controlled to effectively utilize energy generated by the electrical motor using vehicle inertia during braking. Thus efficient storage of generated power is important. On the other hand, the secondary battery must be replaced at fixed intervals generally since the component life of the battery is short. This in turn increases the component running costs.
Tokkai-Hei 6-209501 published by the Japanese Patent Office in 1994 discloses a hybrid drive device in which a condenser is used as a storage device to drive a motor. However a storage device with the required capacity to drive the vehicle has a weight or a volume which exceeds the limited storage space available in the vehicle. This is due to the fact that the condenser has a low energy density.
The present invention has the object of providing a hybrid drive device which efficiently uses the electrical power generated by a generator. A further object of the present invention is to provide a hybrid drive device having low running costs. Yet a further object of the present invention is to provide a hybrid drive device which facilitates storage of components in the limited space available in a vehicle.
A hybrid drive device according to the present invention comprises a generator which is driven by an engine, a storage device which stores generated electrical power from the generator, and an electrical motor which is driven by the electrical power of the storage device and/or the electrical power of the generator. The invention is characterized in that the storage device is provided with a bank of condensers formed by the connection of a plurality of condenser cells in series, and that a parallel monitor is connected in parallel to each condenser cell, the parallel monitor bypassing a charging current when the respective terminal voltages exceed a fixed value.
In this manner, even when the size of the electrostatic capacity or the leakage current of each condenser cell displays large variations, it is possible to charge or discharge each condenser cell equally. In the arrangement provided with a parallel monitor, since it is not necessary to provide for a leeway with respect to a dielectric voltage in consideration of the voltage variation between condenser cells, several tens of percent downsizing of condenser in terms of electrostatic capacity is possible, in comparison with an arrangement not provided with a parallel monitor. Although the dielectric voltage per cell in the condenser is small, the voltage value is increased to a required level by connecting a plurality of such cells in series.
According to the present invention, the storage device is provided with switching converter. The switching converter has fixed current output characteristics and controls a charging current to the condenser bank. Generally a condenser is charged by a fixed voltage power source with half the charging power being lost as heat which results from resistance between the power source and the condensers. In contrast, according to the present invention, the switching converter controls the charging power to the condensers with a fixed current output. As a result, even when charging is performed with a large current which is generated by the electrical motor during regeneration of braking energy, a high charging efficiency of greater than or equal to 90% is achieved. This therefore results in conspicuous improvement to fuel economy performance of the drive device.
Since the condenser is not a fixed voltage device and its state of charge (SOC) is obtained accurately from its voltage, it is possible to perform effective control of the generator based on the state of charge. Furthermore since the charge/discharge cycle life of a condenser is long, component replacement is not required for long periods of time. That is to say, running costs are considerably lower than those associated with a secondary battery.
The hybrid drive device described above can be adapted to a hybrid vehicle. The hybrid vehicle comprises generator which is driven by an engine, a storage device which stores generated power from the generator, and an electrical motor which is driven with the electrical power of the storage device and/or the electrical power of the generator and which drives the drive wheels of the vehicle. The hybrid vehicle is characterized in that a switching converter, a parallel monitor, and a condenser bank are provided as a storage device in the same manner as above. The hybrid vehicle further comprises a controller which detects a state of charge of the storage device and the required drive power of the vehicle and which controls the generated amount of power by the switching converter and the engine.
The controller controls the switching converter so that a motor output is obtained which corresponds to the required drive force. Furthermore the controller controls the engine, that is to say, the generated amount of the generator so that the state of charge is maintained to a suitable value. In this manner, it is possible to maintain a suitable state of charge under normal conditions although the drive device always outputs a drive force which is required by the vehicle.
Furthermore the switching converter displays fixed voltage output characteristics having a wide operating range corresponding to condenser characteristics in which the terminal voltage is reduced to zero as a result of power discharge. This allows more efficient use of stored electrical power. Since the electrical power of the condenser is proportional to the square of the terminal voltage, for example if the switching converter functions up to xc2xd of the rated output voltage, up to 75% of the stored power can be used. Otherwise if the switching converter functions up to xc2xc of the rated output voltage, up to 94% of the stored power can be used. Thus it is possible to obtain a high power utilization efficiency even when performing discharge of large current during acceleration or the like by controlling the operation of the switching converter in response to drive power requirements of the vehicle.
Furthermore mounting of the hybrid drive device in a vehicle is facilitated since the weight and the volume of the storage device are reduced by adapting an electrical double-layer condenser which has a large electrostatic capacity as a condenser cell.